This invention relates to steelmaking and particularly to the making of steel by electric arc furnace (EAF).
The electric arc furnace has evolved considerably with the use of exothermic reactions to complement the electric arc for scrap melting and steel refining. The exothermic reactions have come in two (2) forms: direct oxygen injection using oxygen lances, and the use of oxy-fuel burners. The problem is that each of these energy input systems is efficient for transferring heat in different parts of the campaign, but each must also be controlled to inhibit detrimental interaction between the electrodes, the oxy-fuel burners, and the oxygen lances (as well as to avoid any loss of production or losses of yield efficiency).
Oxy-fuel burners are common equipment in electric arc furnaces. Typically such burners are located in either the slag door or side wall of the EAF. The oxy-fuel burners are very efficient in heat transfer to the scrap at the start of the melt-in period, when the scrap is cold and has a large surface area. As melting proceeds, the efficiency of the oxy-fuel burners drops off dramatically as the scrap surface in contact with the flame decreases and the scrap temperature increases. It is generally recommended that using oxy-fuel burners to transfer heat energy to the melt be discontinued after 50% of the meltdown period is completed (so reasonable steelmaking efficiency is achieved).
Previously, the point at which the oxy-fuel burner use was discontinued was controlled by simply measuring the electricity consumption and cutting off the oxy-fuel burners at a pre-determined setpoint in the melting cycle. An alternative control for the oxy-fuel burners was to measure the temperature of the furnace side panels adjacent to the burners to track burner efficiency and shut off the burners when efficiency drops below a set point. Still another alternative control was measuring the offgas temperature (indicating that more heat is retained in the offgas) and discontinuing the oxy-fuel burner when a set point temperature was reached.
Use of oxygen lances has also become an integral part of EAF steelmaking operations. Water-cooled lances have been used generally for decarborization and dephosphorization of the steel, although in some cases such lances may be used during early melt-in period for scrap cutting as well. Such a conventional water-cooled lance was typically mounted on a platform and penetrated into the sidewall and/or slag door of the EAF furnace through a panel, where the lance sometimes penetrates into the slag layer but not into the molten bath. Consumable lances have also been used to penetrate into the slag layer or the molten bath. These consumable lances are usually composed of consumable pipe which is adjustable as they burn away to give efficient working length.
In more modern EAF furnaces, oxygen lancing has been done through the oxy-fuel burners by simply increasing the oxygen rates and decreasing the natural gas or propane rates through the burners. In any case, melting and refining of the steel during the campaign with the use of separate oxygen lances, or oxygen lancing through the oxy-fuel burners, can be highly efficient. However, such oxygen lancing of any type can negatively impact on productivity, can accelerate electrode erosion, and can have a detrimental effect on the electrodes. Further, oxygen lancing can create unacceptable levels of CO and NOX that must be consumed or otherwise disposed of in the offgas system.
Oxygen lancing takes advantage of exothermic reactions in oxidizing of carbon and iron in the steel melt creating CO and FeO, and in stirring of the molten steel resulting in temperature and composition homogeneity through the molten steel bath. When used for scrap cutting, the oxidation reaction is primarily with the iron resulting in high energy input as iron is reacted to produce FeO. However, this reaction has had a direct negative impact on yield, and therefore high oxygen rates have not been usually used during cut-in of the scrap. Later in the campaign, when a molten pool is formed, the FeO is reduced out of the slag by carbon in the bath. Thus, the net effect is to produce large amounts of CO gas from the oxygen that is injected by lancing.
The oxygen rate during lancing must therefore be controlled so as not to create excessive yield losses and generate high levels of CO that must be captured in the DES system and in unacceptable ambient levels in the work environment. Typically up to 10% of the CO not oxidized in the furnace is exhausted through the secondary fume capture system during meltdown of the steel. Moreover, the major drawback in high oxygen lance rates is the effect on fume system controls and the production of NOX. All of this must be taken into account in controlling and varying oxygen lance rates during the campaign, and balancing the natural gas or propane levels with the oxygen rates where the oxygen lancing and the oxy-fuel burning is done through the same orifices.
Furthermore, the carbon that reacts with the oxygen to form CO in the molten steel bath can be from the electrodes, increasing electrode erosion and melting costs. Typically carbon is injected into the bath to provide for decarbonization of the molten steel, but also to protect electrodes from such erosion. If in addition, the stirring effect of the oxygen lancing brings bath carbon or injected carbon into contact with the FeO in the slag, an even greater quantity of CO can be produced.
The carbon injection must be controlled for this purpose, as well as to cause foaming of the slag. The foaming of the slag has great benefits in operation of the electric arc furnace in greatly reducing heat loss to the side walls of the furnace, and channelling heat transfer from the electric arcs to the molten steel thereby providing for higher rates of energy input, reduced power and voltage fluctuations, reduced electrical and audible noise, and increased arc length without increasing heat loss, electrode consumption, or refractory consumption.
There is needed, therefore, a simple and efficient system for coordinating the operation of the oxy-fuel burning, oxygen lancing, carbon injection, and electrical current input so as to provide high productivity steelmaking in the EAF. The present invention provides just such a system.
The present invention involves a method of making steel in an electric arc furnace comprising the steps of: measuring electrical current in at least two of the three (3) phases of electric power supplied to the electrodes in an electric arc furnace during a campaign, and establishing at least one setpoint level for the measured electrical current and input time intervals for said setpoint current level for switching between operating modes of inputting exothermic energy or electrical energy, or both, to a steel melt in the electric arc furnace during a campaign. The switching between the modes of operation of inputting exothermic energy or electrical current, or both, is based upon measuring the electrical current in at least two of the three phases of electric power input to the electrodes of the electric arc furnace during a campaign and switching between operating modes when the setpoint current levels for setpoint time intervals for switching are measured.
For inputting exothermic energy to the steel melt in the electric arc furnace during a campaign, at least one setpoint for the measured electrical current and input time interval for said setpoint time interval for switching are established preferably for three operating modes of inputting exothermic energy. The first operating mode is to input exothermic energy to the steel scrap in the furnace following charging by combustion of a combustion fuel/oxygen mixture flow through one or more oxy-fuel burners. The combustion fuel may be natural gas, propane or fuel oil or another form of combustion fuel, and the oxygen is commercial grade oxygen generally about ninety eight percent (98%) purity, or better. The mixture in the first operating mode is high in combustion fuel and the oxygen to combustion fuel ratio is sufficient to combust at least most of the combustion fuel to melt scrap or other iron sources charged to the furnace. The ratio of the mixture of oxygen to combustion fuel during this operating mode may be about 2 to 1, or greater.
In the second operating mode, exothermic energy heat is inputted to the partially melted scrap or other iron sources in the electric arc furnace by combustion of a combustion fuel/oxygen mixture where the mixture has a reduced flow of combustion fuel and increased flow of oxygen, and is capable of cutting a hole through scrap to provide a path for oxygen injection into the melt. In the second mode, the ratio of oxygen to combustion fuel may be about 5 to 1, or greater.
In the third operating mode of inputting exothermic energy to the steel melt in the electric arc furnace during the campaign, oxygen is injected into the molten metal bath in the furnace to provide for decarbonization, dephosphorization and refining. The oxygen may be injected through separate oxygen lances, but preferably is injected through the oxy-fuel burners used in the first and second operating modes of inputting exothermic energy to the steel melt. In the third mode, where the oxygen is injected through the oxy-fuel burners, the ratio of oxygen to combustion fuel is at least about 11 to 1 and more desirably about 28 to 1.
Alternatively, or in addition to the control of exothermic energy input to the furnace, the electrical current in at least two phases of the three-phase electric power transferred to the electrodes in the furnace can be measured during a campaign, and the modes of operation of the electric arc furnace switched during the campaign based on the measured current levels reaching setpoint levels for set time intervals during the campaign. The first operating mode of inputting electrical current through the electrodes into the scrap within the furnace is a short arc as the electrodes are lowered into the furnace. The second operating mode inputting energy in the form of electrical current into the steel melt is a long arc primarily to melt the scrap and other sources of iron in the furnace. The third operating mode of inputting electrical current through the electrodes into the molten steel bath in the furnace is in a short arc to increase the temperature of the molten steel and to provide for decarbonization, dephosphorization, slag foaming and other refining.
The input setpoint levels of measured electrical current, and of the input time intervals, for switching between operating modes of inputting exothermic energy and of inputting electrical energy through the electrodes into the steel melt in the furnace may be independently set, or in some cases, may be the same. For example, the input setpoint electrical current levels and independent time intervals for switching between the first operating mode and the second operating mode of inputting exothermic energy may be the same or different than the setpoint levels of measured electrical current and time intervals for switching between the second operating mode and the third operating mode of inputting exothermic energy into the steel melt in the electric arc furnace during a campaign. Similarly, the setpoint levels of measured electrical current, and the time intervals for those current levels, for switching between the first operating mode and the second operating mode of inputting electrical energy through the electrodes to the steel melt in the electric arc furnace may be the same or different from the setpoint current levels and time intervals for switching from the second operating mode to the third operating mode of inputting electrical energy through the electrodes into the steel melt in the furnace. Also, the input setpoint electrical current levels and time intervals for switching between operating modes of inputting exothermic energy and of inputting electrical energy into the steel melt in the electric arc furnace may be the same or different.
The measured electrical current of the electric energy input during the three operating modes may be indirectly measured by a regulator, such as an AMI xe2x80x9cDigiarcxe2x80x9d, that measures standard deviation of electrical current of the measured phases of electric power. Alternatively, the electrical current may be directly measured in at least two of the three phases or otherwise indirectly measured, for example, in the form of power or other units. In any case, the switching between operating modes of inputting exothermic energy and the operating modes of inputting electrical energy through the electrodes into the steel melt in the electric arc furnace during a campaign may be done automatically based on the electrical current values measured, directly or indirectly, in at least two of the three phases of electric power input. In any event, the operation of the electric arc furnace may be switched back to the previous mode of operation manually or automatically if the measured electrical current values increase above the same, or a separate setpoint levels, for that operating mode for the same or a different set time interval.
Carbon may also be injected into the electric arc furnace in the operation of the present invention manually, or automatically, as the modes of operation of inputting exothermic energy into the steel melt proceed. The carbon may be injected at a low rate manually or automatically during the second operating mode of exothermic energy input as required for slag foaming and melt-in carbon control, and the carbon may be injected at a higher rate during the third operating mode of inputting exothermic energy as required for slag foaming and decarbonization of the steel melt. The injection of carbon may be automatic during the third operating mode of exothermic energy input, as well as adjustable by the operator as required for slag foaming and decarbonization of the steel melt. The carbon may be injected in the second mode of exothermic input at a rate of at least one pound per minute, and in the third mode of exothermic energy input at a rate of at least one pound per minute. Carbon may also be injected in the first mode of exothermic energy input, generally manually, as melting and refining conditions may require.
Other aspects of the present invention as well as its benefits in operation will become apparent as the description of the following embodiments of the present invention proceed.